Proposed commit log message:

[NEW PORT] sysutils/py-cron-descriptor: Converts cron expressions into human readable strings

A Python library that converts cron expressions into human
readable strings. Ported to Python from
https://github.com/bradyholt/cron-expression-descriptor.

WWW: https://github.com/Salamek/cron-descriptor

Reviewed_by: koobs
Approved by: koobs (mentor)
Differential_Revision: D23018

• portlint: OK (looks fine.)
• testport: OK (poudriere: 1300069, [armv7, aarch64], 'BUILD_ALL_PYTHON_FLAVORS=1', [py27, py35, py36, py37, py38] tested)
• unittest: OK (

Python2.7 on armv7: Ran 177 tests in 0.449s OK
Python3.5 on armv7: Ran 177 tests in 0.520s OK
Python3.6 on armv7: Ran 177 tests in 0.604s OK
Python3.7 on armv7: Ran 177 tests in 0.542s OK
Python3.8 on armv7: Ran 177 tests in 0.553s OK
)